The biggest thing you need to worry about is memory leaks. some of them HAVE HUGE memory leaks where your memory will drain till you have to reboot. BBCrackman's OS V5.1 manages memory the best in my opinion. with some other OS's if you go below 20 it begins to get extremely sluggish and you have to reboot.

With V5.1 you can get down to 10 or below then you would have to reboot.

Alot of people like the theme VISTA COMPLETE and it has the most features and coolest effects (double hidden sliding dock) but it leaks memory like crazy. It will leak all the way down in a few hours.